High-dielectric material

ABSTRACT

A bulk dielectric material comprises a solid composite material comprising a solid matrix material and a plurality of filler elements distributed within the matrix material. The bulk dielectric material has, at a frequency greater than 1 MHz, (i) a permittivity having a real part of magnitude greater than 10 and an imaginary part of magnitude less than 3, and (ii) an electrical breakdown strength greater than 5 kV/mm and has a minimum dimension greater than 2 mm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/594,569, filed Apr. 4, 2008, which is a national phase application ofPCT application PCT/GB2008/001185, filed Apr. 4, 2008, which claimspriority to GB 0706638.4 filed Apr. 4, 2007. All applications areincorporated herein by reference in their entirety and for all purposes.

FIELD OF THE INVENTION

This invention relates to high-dielectric materials; more particularly,the invention relates to materials having an electrical permittivitywith a high real part and a low imaginary part (i.e. a low loss), andhaving a high electrical breakdown strength.

BACKGROUND ART

The electrical breakdown strength of a material is the magnitude ofelectric field required to cause the material to conduct (typicallymeasured as the potential difference at breakdown between two electrodesin contact with opposite sides of a sample of the material, divided bythe electrode separation).

Electrical permittivity {circumflex over (∈)}(ω) of a material isdefined as:

D ₀ e ^(iωt)={circumflex over (∈)}(ω)E ₀ e ^(iωt)

where E₀ and D₀ are the amplitudes of an electric field and acorresponding displacement field in the material (respectively), ω isthe angular frequency of the fields, t is time, and i is the square rootof minus 1. {circumflex over (∈)}(ω) is a complex number: the real partis related to the refractive index seen by electromagnetic waves in thematerial; the imaginary part is related to the dielectric lossexperienced by electromagnetic waves in the material. {circumflex over(∈)}(ω) is also called the frequency-dependent dielectric constant; itsdc value (i.e. its value at a frequency of zero) is known as the staticdielectric constant. Measuring the magnitude of the permittivity and theloss tangent (for example, by measuring the capacitance and conductanceof a parallel-plate capacitor sandwiching the material being tested)enables ready calculation of the real and imaginary parts of thepermittivity. The square of the magnitude of the permittivity|{circumflex over (∈)}(ω)| is of course equal to the sum of the squaresof the real part {circumflex over (∈)}(ω)_(real) and the imaginary part{circumflex over (∈)}(ω)_(imag) of the permittivity {circumflex over(∈)}(ω), i.e. {circumflex over (∈)}(ω)_(real) ²+{circumflex over(∈)}(ω)_(imag) ². The loss tangent is the ratio of energy dissipated toenergy stored in the dielectric material. The loss tangent equals theimaginary part of the permittivity divided by the real part, i.e.

$\frac{{\hat{ɛ}(\omega)}_{imag}}{{\hat{ɛ}(\omega)}_{real}}.$

In our experiments (described below), we measured, using an Agilent4285A 75 kHz-30 MHz Precision LCR Meter, the magnitude of thepermittivity |{circumflex over (∈)}(ω)| and the loss tangent of samplesheld in a sample holder similar to that shown in FIG. 4 (though not inan oil bath).

Applications exist for mechanically robust material systems that exhibithigh electrical breakdown strength, and have a high dielectric constant,at frequencies ranging from a few MHz to GHz and low dielectric loss.For example, a material with a higher dielectric constant can be madeinto a device (for example, a lens) that is smaller than a lens with thesame properties (e.g. focal length) made from a material with a lowerdielectric constant. That is because the electromagnetic “path length”per unit physical length is higher in the higher-dielectric materialthan in the lower dielectric material; the phase of an electromagneticwave propagating in the material progresses by a given amount over ashorter distance in the higher-dielectric material.

Most materials that are currently used for applications requiring highdielectric constants are either low-loss ceramics or liquids. Whilstboth exhibit reasonable electrical performance, they are notmechanically robust, as sintered ceramics are brittle and liquid mixescan only be used in a limited range of environments and configurations.Also, many high-dielectric-constant materials exhibit high breakdownstrength (BDS) when in film form (i.e. when they have thicknesses of afew microns or tens of microns) but when they are in bulk form the BDSis much reduced.

An object of the invention is to provide a dielectric material havingdielectric properties similar to or better than prior-art ceramic andliquid dielectric materials and having better mechanical properties thanthose materials.

DISCLOSURE OF THE INVENTION

In a first aspect, the invention provides a bulk dielectric materialhaving, at a frequency greater than 1 MHz, (i) a permittivity having areal part of magnitude greater than 10 and an imaginary part ofmagnitude less than 3, and (ii) an electrical breakdown strength greaterthan 5 kV/mm, the bulk dielectric material being a solid compositematerial, comprising a solid matrix material and a particulate fillermaterial within the matrix material, and having a minimum dimensiongreater than 2 mm.

The invention thus provides a bulk material with a dielectric constant,electrical breakdown strength and dielectric loss comparable with orbetter than prior-art ceramic and liquid high-dielectric materials. Useof a bulk composite material offers improved mechanical propertiescompared with prior-art materials. For example, liquids (eg glycol,water) need housing but are corrosive to metallic housings and leachplasticisers and impurities from plastic housings; consequently, theyhave a limited lifetime, and although they need to be of high puritythey are easily contaminated. Moreover, leaching or corrosion or anyother form of contamination of the liquids leads to impairment ofbreakdown strength.

The invention also provides an object comprising or consisting of thebulk dielectric material. Some prior-art composite materials achievehigh breakdown strengths when in film form, but not in bulk form. Theinvention provides a bulk dielectric material having a permittivity andbreakdown strength that are only achieved in thin film form by prior artmaterials. The material may be a block, which may be of any shape,including for example a sheet. The material has a minimum dimensiongreater than 2 mm, that is it is a bulk material measuring greater than2 mm in all directions. The bulk dielectric material may have a minimumdimension greater than 3 mm, preferably greater than 5 mm, morepreferably greater than 10 mm and yet more preferably greater than 20mm.

Materials according to the invention may have still better electricalproperties: the real part of the permittivity may be greater than 15,preferably greater than 20, still more preferably greater than 25 oreven greater than 30; the permittivity may have an imaginary part ofmagnitude less than 2, preferably less than 1, preferably less than 0.8,preferably less than 0.5, still more preferably less than 0.3, less than0.1, or even less than 0.05; the electrical breakdown strength may be6.5 kV/mm or more, preferably greater than 10 kV/mm, preferably greaterthan 15 kV/mm, preferably greater than 20 kV/mm, still more preferablygreater than 25 kV/mm, or even greater than 30 kV/mm.

The real part of a material's permittivity generally tends to fall withincreasing measurement frequency. The material may have the specifiedpermittivity and electrical breakdown strength at frequencies of lessthan 100 GHz, less than 75 GHz, less than 40 GHz, less than 30 GHz, lessthan 20 GHz, or less than 10 GHz. The material may have the specifiedpermittivity and electrical breakdown strength at frequencies of morethan 5 MHz, more preferably greater than 10 MHz and yet more preferablygreater than 20 MHz.

The breakdown energy density

$U_{breakdown} = {\frac{1}{2}{\hat{ɛ}(\omega)}V_{breakdown}^{2}}$

can be another indicator of the electromagnetic properties of thecomposite material. It may be that the breakdown energy density,normalised to the breakdown energy density of the filler material, isgreater than 1, preferably greater than 1.2, more preferably greaterthan 1.4, still more preferably greater than 1.6, still more preferablygreater than 1.8, or even greater than 2.0.

However, it should be noted that, for some applications, it may not bedesirable to have the highest possible real part of the permittivity.For example, a particular application may require the bulk dielectricmaterial to provide a given electromagnetic path length over a fixedphysical length; similarly, the bulk dielectric material may be used ina capacitor required to provide a given capacitance with a given plateseparation. In those and other cases, it may be that the bulk dielectricmaterial is required to have a real part of its permittivity that ishigher than that of prior-art dielectric materials, but not as high ascan be achieved; thus, the real part of the permittivity may fallbetween two of the limits specified above (e.g. between 10 and 15).

Preferably the bulk dielectric material has good mechanical properties.For example, its ultimate strain to failure (i.e. the maximum extensionit can withstand before failing, as a percentage of its length) may begreater than 0.5%, preferably greater than 0.6%, more preferably greaterthan 0.7%, still more preferably greater than 0.8%, or even greater than0.9%. It may be that before failure it can absorb more than 100 kJ/m³,preferably more than 200 kJ/m³, more preferably more than 300 kJ/m³,still more preferably more than 400 kJ/m³, or even more than 440 kJ/m³.

The matrix material may be an organic resin. The matrix material may bea polymer material (e.g. a silicone). The matrix material may be ahomopolymer. The matrix material may be a urethane polymer.

The matrix material may be a thermosetting material. The matrix materialmay be UV cured.

The matrix material may be a ceramic.

The matrix material may have a lower dielectric constant than the fillermaterial; the filler material may thus have a higher dielectric constantthan the matrix material.

The matrix material may have, at a frequency greater than 1 MHz, (i) apermittivity having a real part of magnitude greater than 10 and animaginary part of magnitude less than 3, and (ii) an electricalbreakdown strength greater than 5 kV/mm. The matrix material may havestill better electrical properties: the real part of the permittivitymay be greater than 15, preferably greater than 20, still morepreferably greater than 25 or even greater than 30; the permittivity mayhave an imaginary part of magnitude less than 2, preferably less than 1,preferably less than 0.8, preferably less than 0.5, still morepreferably less than 0.3, less than 0.1, or even less than 0.05; theelectrical breakdown strength may be 6.5 kV/mm or more, preferablygreater than 10 kV/mm, preferably greater than 15 kV/mm, preferablygreater than 20 kV/mm, still more preferably greater than 25 kV/mm, oreven greater than 30 kV/mm. The matrix material may have any of theelectrical or mechanical properties of the bulk dielectric materiallisted herein.

The filler material may have, at a frequency greater than 1 MHz, (i) apermittivity having a real part of magnitude greater than 10 and animaginary part of magnitude less than 3, and (ii) an electricalbreakdown strength greater than 5 kV/mm. The filler material may havestill better electrical properties: the real part of the permittivitymay be greater than 15, preferably greater than 20, still morepreferably greater than 25 or even greater than 30; the permittivity mayhave an imaginary part of magnitude less than 2, preferably less than 1,preferably less than 0.8, preferably less than 0.5, still morepreferably less than 0.3, less than 0.1, or even less than 0.05; theelectrical breakdown strength may be 6.5 kV/mm or more, preferablygreater than 10 kV/mm, preferably greater than 15 kV/mm, preferablygreater than 20 kV/mm, still more preferably greater than 25 kV/mm, oreven greater than 30 kV/mm. The filler material may have any of theelectrical or mechanical properties of the bulk dielectric materiallisted herein.

The filler material may be solid. The filler material may comprise aceramic material. The ceramic material may be a titanate, for exampleSrTi0₃. The ceramic material may be a niobate, for example leadmagnesium niobate.

The filler material may comprise solid particles. The particles of thefiller material may have a diameter of less than 250 microns, less than200 microns, less than 150 microns, less than 100 microns, or even lessthan 50 microns. The particles of the filler material may have agenerally rounded shape.

Alternatively, the filler material may comprise a fluid. The fluid maybe an encapsulated fluid (e.g. a microballoon).

Thus, known high-dielectric material ceramics or fluids—used in bulk inthe prior art—may be used as the filler material in example embodimentsof the present invention.

The particles of the filler material may comprise a coating. Asdiscussed below, coating the filler material particles can producesignificant improvements in electrical and mechanical performance. Thecoating may be a multilayer coating. The coating may be a modifiedsurface of the filler material particles. The coating may be anorganosilicate layer. The coating may be an epoxy layer. The coating maycomprise vinyl groups. The coating may be made from a urethane coating.The coating may be derived at least in part from styrene. The coatingmay be derived at least in part from ethylene. The coating may comprisea metal or metal oxide.

The particles of the filler material may contain pores. The coatingmaterial may penetrate into the pores.

It will be understood that a large number of particles of the fillermaterial are distributed throughout the matrix material.

The particles of the filler material may be distributed substantiallyhomogeneously throughout the matrix material. A material withhomogenously distributed filler particles may generally be treated as ifit were a bulk material having the effective (i.e. net) mechanical andelectrical properties produced by the combination of the matrix materialand the filler material. Alternatively, the particles of the fillermaterial may be distributed inhomogeneously throughout the matrixmaterial. A material having inhomogeneously distributed filler particlesmay be a bulk material that displays localised effects (for examplelocalised electromagnetic effects, for example localised focusing orother electromagnetic-field-shaping effects).

The bulk dielectric material may comprise more than 40%, more than 45%,more than 50%, more than 60%, or even more than 65% filler material byvolume. The bulk dielectric material may comprise less than 50%, lessthan 55%, less than 60%, less than 65% or less than 70% filler materialby volume. Preferably, the bulk dielectric material comprises 40% to 70%of the filler material by volume. Still more preferably, the bulkdielectric material comprises 50% to 65% filler material by volume.

According to a second aspect of the invention there is provided a methodof producing a bulk dielectric material having, at a frequency greaterthan 1 MHz, (i) a permittivity having a real part of magnitude greaterthan 10 and an imaginary part of magnitude less than 3, and (ii) anelectrical breakdown strength greater than 5 kV/mm, the methodcomprising the step of forming the bulk dielectric material bydistributing a particulate filler material in a matrix material to forma solid composite material, having a minimum dimension greater than 2mm.

The method may comprise the step of mixing the filler material with thematrix material at a temperature above room temperature.

The method may comprise the step of compressing a mixture of the fillermaterial and the matrix material.

The mixture may be compressed at a pressure of at least 1 GPa. A mixtureof the filler material and the matrix material may be placed within acontainer and the container heated and pressurised (for example, thecontainer may be an autoclave).

A mixture of the filler material and the matrix material may be placedwithin a container and the container evacuated, for example toinfiltrate the matrix material into the filler material.

The matrix material may be a non-thermosetting polymer, and the fillermaterial may be mixed with the polymer.

Alternatively, the matrix material may be a thermosetting polymer, andthe filler material may be mixed with the polymer or with a precursormaterial (e.g. a monomer) that is cured to form the polymer.

Advantageously, the viscosity of the matrix material is sufficiently lowto enable effective infiltration of the filler material. Of course, itwill be appreciated that the viscosity of the matrix material may betemperature dependent.

Advantageously, the matrix material may have a sufficient ‘working life’(i.e. curing time) to allow thorough degassing of the matrix material.

The method may comprise the step of providing the particles of thefiller material with a coating. The coating may be a modified surface.The coating may be an organosilicate shell. The coating may be appliedby silylation. The coating may be an epoxy shell. The coating maycomprise vinyl groups. The particles of the filler material may containpores and the coating may penetrate into the pores.

The coating may comprise a chemical functionalisation of the particulatesurface. The functionalisation may aid the wetting of the resin andimprove the homogeneity of the samples.

The method may comprise the step of providing the particles of thefiller material with a metal or a metal oxide. For example, the fillerparticles may be coated with silver, aluminium, or aluminium oxide.Loading the filler particles with a metal or metal oxide in that mannerincreases the net permittivity of the filler particles. The fillerparticles may comprise 1% or more, 2% or more, 5% or more, 8% or more,or 10% or more metal or metal oxide, by weight.

The coating may be applied under a vacuum.

The method may comprise building up the bulk dielectric material in aplurality of layers. Each layer may be of a thickness greater than 5 mm,or greater than 10 mm. Each layer may be of a thickness less than 5 mm.

It may be that the bulk dielectric material is in all directions thickerthan 3 mm, preferably thicker than 5 mm, more preferably thicker than 10mm, still more preferably thicker than 15 mm and yet more preferablythicker than 20 mm.

The bulk dielectric material may be a sheet material (but it is not athin film).

The method may comprise the bulk dielectric material being cast in amould having a minimum dimension greater than 5 mm. The mould may have aminimum dimension greater than 10 mm, greater than 15 mm or even greaterthan 20 mm. The casting may occur in a single step; it may be a‘one-shot’ casting.

The casting may occur under a vacuum.

According to a third aspect of the invention, there is provided a devicecomprising a bulk dielectric material according to the first aspect ofthe invention or a bulk dielectric material made by the method of thesecond aspect of the invention. The device may utilise anelectromagnetic field and the bulk dielectric material may be used toalter the behaviour of the electromagnetic field. It will be understoodthat the bulk dielectric material will be useful in a wide range ofdevices; examples include a capacitor, a lens, an oscillator or atransmission line.

According to a fourth aspect of the invention, there is provided use ofa bulk dielectric material according to the first aspect of theinvention or a bulk dielectric material made by the method of the secondaspect of the invention to alter the properties of an electromagneticfield. The electromagnetic field is altered in the sense that at somepoint in space it takes a different value from that which it would haveif the dielectric material were not used. The electromagnetic field maypass through the bulk dielectric material. The use may be use in adevice, in which the bulk dielectric material is comprised.

According to a fifth aspect of the invention, there is provided anarticle consisting of a bulk dielectric material according to the firstaspect of the invention. It may be that the article is in all directionsthicker than 3 mm, preferably thicker than 5 mm, more preferably thickerthan 15 mm and still more preferably thicker than 20 mm.

According to a further aspect of the invention, there is provided adielectric material having (i) a permittivity measured having a realpart of magnitude greater than 10 and an imaginary part of magnitudeless than 1, and (ii) an electrical breakdown strength greater than 5kV/mm, characterised in that the dielectric material is a bulk solidcomposite material, comprising a solid matrix material and a particulatefiller material within the matrix material.

According to a still further aspect of the invention, there is provideda method of producing a dielectric material having (i) a permittivityhaving a real part of magnitude greater than 10 and an imaginary part ofmagnitude less than 1, and (ii) an electrical breakdown strength greaterthan 5 kV/mm, characterised in that the method comprises the steps offorming the dielectric material by distributing a particulate fillermaterial in a matrix material to form a bulk solid composite material.

It will be appreciated that aspects of the present invention describedin relation to the method of the present invention are equallyapplicable to the material of the present invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain illustrative embodiments of the invention will now be describedin detail, by way of example only, with reference to the accompanyingdrawings, in which:

FIG. 1 is (a) a method (Method A) of manufacturing a compositedielectric material and (b) to (e) four methods (Methods B to E) ofmanufacturing examples of composite materials according to exampleembodiments of the invention;

FIG. 2 shows microstructure of (a) a composite material according to anexample embodiment of the invention, manufactured by the method shown inFIG. 1( c) (Method C) and (b) a composite material manufactured by themethod of FIG. 1( a) (Method A);

FIG. 3 is an example of a disk of bulk dielectric material according toan example embodiment of the invention, used in testing of thematerial's electrical breakdown strength;

FIG. 4 is apparatus used to test the electrical breakdown strength ofthe disk of FIG. 3;

FIG. 5 is a plot of permittivity against volume fraction of fillerelements in matrix material for dielectric material made by the methodsof FIG. 1, showing (a) the real part and (b) the imaginary part;

FIG. 6 shows (a) and (b) microstructure of a composite materialmanufactured by the method of FIG. 1( a)(Method A);

FIG. 7 is a plot against permittivity of breakdown strength in kV/mm fordielectric materials made by the methods of FIG. 1 and by the method ofFIG. 1( b), some including an additional silylation step (Method C′) andsome an additional autoclaving step;

FIG. 8 is a plot of load in Newtons against displacement in mm ofsamples of pure K200 ceramic, a composite dielectric material made byMethod C of FIG. 1, and a composite dielectric material made by MethodC′;

FIG. 9 is a chart showing the absorbed energy density at breakdown, thatbeing the areas under the curves of the plot of FIG. 8;

FIG. 10 is the plot of FIG. 7 without results for the autoclavingmethods but with additional data, for the breakdown strength andpermittivity of lead magnesium niobate and K1700 material.

DETAILED DESCRIPTION

The manufacturing methods used in our experiments can be divided intothree groups. The three groups are described briefly in the next threeparagraphs before being described in detail below.

The materials of the first group were fabricated according to a firstmethod (Method A). Materials within this group are not materialsaccording to the present invention.

The materials of the second group were made by a variety of methods(Methods B to E). Materials within this group generally had improvedelectrical and/or mechanical properties, compared with the materialsproduced by Method A and materials of the prior art. Materials withinthis group are examples of materials according to the invention.

The materials of the third group were made by a sixth method (MethodC′), which was Method C with an additional silylation step. Materialswithin this group had still better properties than the materials of thefirst two groups and are examples of material according to theinvention.

Values given herein that vary with electromagnetic frequency are basedon measurements made at point frequencies of 75 KHz, 500 KHz, and 1 MHz(the data shown in the accompanying Figures was obtained at 1 MHz).Values were also measured from 1 MHz through to several GHz (about 12GHz) and the values change by a few percent, (typically 1-5%) over thatrange.

In our preliminary experiments, we identified a ceramic material, “K200”as a candidate material for use as a particulate filler material indielectric material according to an example of the invention. The K200material is a calcium titanate-based proprietary ceramic material,Morgan Electro Ceramic High Voltage capacitor material K200, obtainedfrom Morgan Electro Ceramics, Vauxhall Industrial Estate, Ruabon,Wrexham, LL14 6HY. The K200 ceramic was obtained in both sintered powderform and consolidated block form. The block was ground down to aparticulate form using standard techniques.

The K200 material was dispersed in matrix materials that had highbreakdown strength, good mechanical properties, low dielectric loss, andoffered ease of processing. Specifically, we selected two epoxy-basedresin systems, Struers Epofix epoxy Resin (Cat. No. 40200029) and Robnor(PX900D/NC) resin and a siloxane-based material, Dow Corning Sylgard 184silicone elastomer, for further investigation.

In method A (FIG. 1( a)), samples were manufactured by first mixing theK200 particulates with matrix material. The mixture was diluted with avolatile miscible solvent. The solvent was evaporated once good mixingwas achieved. The resulting, highly viscous mix was compacted under 1.25GPa of pressure (that is, 10 tonnes applied to a region of 1 cmdiameter) and then cured for one hour at 100 degrees C., in moulds whichprovided the sample shape required for our electromagnetic measurements.Those measurements showed (FIG. 5 (a) and (b), data marked with squares)that the K200 material, when dispersed in a polymeric matrix, exhibitedreasonably high real permittivity values whilst displaying low imaginaryvalues (i.e., low loss).

The materials exhibited the behaviour one would expect as the volumefraction of filler material increases: the real and imaginary parts ofthe complex permittivity increased with volume fraction. In order toachieve high dielectric constants (i.e. high real parts of thepermittivity), it was necessary to go to high volume fractions, whichalso meant high losses.

However, we believed that the major reason for the high loss lay in thedifficulties involved in preparing high quality samples at high volumefractions. High filler content led to very viscous mixtures and, duringthe mixing phase, air became entrained. That resulted in the formationof voids, which became more evident at the higher volume fractions,where the mix became more difficult to process.

We developed and investigated a variety of techniques in an effort toensure good dispersion and homogeneous mixing of the filler material inthe matrix systems, whilst reducing the void content. We found that nosingle method was optimal over the whole range of filler volume fractionthat we wished to investigate; thus a range of methods were developed,each of which was best suited to a particular range of volume fraction(VF); the volume fractions preferred for each method were typically

Method A-65-70 vol %

Method B-0-50 vol %

Method C-50-65 vol %

Method D-45-55 vol %

Method E-60-65 vol %

We expect that those ranges will be widened as our manufacturingtechniques improve.

The methods are shown schematically in FIG. 1( b) to (e). A briefdescription of the methods is given below, together with theirlimitations.

Method B (FIG. 1( b)) relied on increasing the viscosity oflow-volume-fraction systems by thermally initiating gelation of theresin. The increase in viscosity prevented the dense ceramic materialfrom settling out of suspension. The filler material particulates weremixed with the matrix material at 60 degrees C. When the resulting mixbecame highly viscous, it was compacted under 1.25 GPa of pressure. Thematerial produced by this method had a relatively low VF.

In Method C (FIG. 1( c)) the filler material particulates were placed ina mould and compacted (by hand). Degassed resin and hardener were added.The mixture was then placed in an evacuation chamber. The chamber waspumped (evacuated) to promote resin infiltration of the matrix material.The resulting material had a medium VF.

Method D involved pre-mixing the polymer and ceramic prior to carryingout the steps of Method C (excluding the compaction step).

In Method E, the filler powder was pre-mixed with resin, prior tocompaction between frits and then vacuum resin infiltration; highcompaction pressures were employed (10 tonnes). The resulting materialhad medium to high VF.

During the development of these methods attention was given to the needfor each of them to be scaled-up, since it was desired that sampleslarge enough for high-voltage testing could ultimately be prepared.

FIG. 2 shows optical micrographs of resin-K200 composite materials. FIG.2( a) shows the excellent dispersion of ceramic in resin that can beachieved using Method C′. In contrast, the initial simple mixing method(method A) resulted in a composite such as that shown in FIG. 2( b),where large voids and resin rich (i.e ceramic free) areas can be seen.

During characterisation of the K200 material we noted that there was avariation of particle size within the bulk. Particles fell into therange of around 50 microns to 200 microns. In order to investigatewhether the particle size had any significant influence over the finalelectromagnetic properties of ceramic/resin composites we elected tofractionate the powder using standard grinding and sieving procedures.The following particle-size fractions were obtained: less than 53micron, 53 to 90 micron, 90 to 125 micron, 125 to 212 micron, andgreater than 212 micron. Composites were prepared and electricallytested. Although there was a variation in the loss values, it was notsignificant; thus we concluded that, in the range of particle-sizes thatwe have investigated, the particle size had no significant effect on thepermittivity in the case of this specific material system. (However, wedid not at this stage investigate silylation or other coating of theseparticles, which could prevent them from clumping, aggregating oragglomerating.)

In order to undertake high-voltage testing, sample discs of compositedielectric material were prepared. The discs had a thickness of 2 mm to3 mm and were of 36 mm diameter (FIG. 3). The samples werelapped-grinded post-manufacture to ensure that they had flat andparallel surfaces.

Electrodes of 16 mm size were provided on each face. The electrodes hadto be centrally located on each face and they had to be verticallyaligned. A variety of metals such as silver, gold and gold-palladiumalloy were explored; we used silver paste (Acheson silver DAG 1415M)aluminium tape (3M Scotch Brand Aluminium tape 0.09 mm thick), andsputtered gold-palladium using magnetron sputterer. Since themetallization needed to be highly conducting, be well adhered to thesubstrate, and be compliant in the case of the Sylgard matrix (aflexible substance), we elected to use a screen printing method thatdeposited the conductor via aligned masks.

A test-rig for the high voltage measurement was purpose built. FIG. 4shows a sample under test. The high voltage probe, the resistor voltagedivider network, and the sample in the oil bath can be seen.

The dielectric properties (at 1 MHz) versus volume fraction of thelarge-scale samples are plotted in FIG. 5. The plot illustrates thatlarge-area samples with high volume fraction and high values ofpermittivity were manufactured. It is important to note that highimaginary permittivity values (i.e. high losses) at high volumefractions are again seen in the data of FIG. 5. However the use ofmethods C and C′ (i.e. using degassing and silylation) has allowedreduction of the imaginary permittivity to values less than 1, typically0.02 in the case of Method C′.

The electrical breakdown strength (BDS) of the samples fabricated bymethod A was found to be below 5 kV/mm, with values decreasing withincreasing permittivity. We believe that the relatively low BDS was aresult of voids present in the material, which increase in number athigher VF with the loading of filler (i.e. with increasingpermittivity). FIG. 6( a) shows porosity in the samples obtained bymethod A and we believe that to have been a major contributor to the lowBDS we observe. In addition, we also believe that a second mechanism isat least partly responsible for the reduction in BDS with increasingpermittivity: that of “percolation” where the particles are inelectrical contact. FIG. 6( b) shows a micrograph that illustrateschains of closely connected particles that could lead topercolation-induced electrical breakdown.

We believed at this stage that further improvements in electricalcharacteristics of the dielectric materials would only be achieved by amajor deviation from the approach we were using.

We decided to undertake processing of the base K200 particulates with aview to coating the materials with functional groups. The rationalebehind this was to improve the wetting of the K200 to the epoxy moiety;that would then lead to the material being more processable, allowinghigher volume fractions to be dispersed within the matrix and higherpermittivity values to be achieved. Furthermore the coating wouldprovide electrical isolation between the particles and hence improve theBDS.

We therefore undertook a process of silylation that resulted in the K200surface being coated with a shell of aminosiloxane terminated with NH2groups (i.e. the surface is modified). This can be representedschematically as follows:

The NH2 group reacts and bonds to the epoxide group as shown thus:

We prepared samples of this material (in Method C′, being method C withthe silylation step) and characterised their breakdown strength andpermittivity. A detailed description of the method employed is asfollows:

Silylation of Titanate Ceramics

K200 titanate ceramic powder (1200 g) was lightly ground using a pestleand mortar in order to break down any aggregates. The powder was addedto a mechanically stirred solution of 3-aminopropyltriethoxysilane (100g) in ethanol (2.5 L) contained in a glass round bottomed three neckedflask. The flask was stoppered and the mixture was stirred for a day(˜24 h) at room temperature. The mixture was filtered and the collectedsolid was washed with ethanol (3 times 250 ml) then it was dried in airin a fume extracted cabinet. Finally, the powder was placed in an ovenat 110 C for two hours and was then allowed to cool before use.

Note that methanol can be used as a replacement for ethanol withoutdetriment to the reaction. The silylation reaction requires the presenceof small quantities of water in order to proceed; however, the reagentgrade alcohol used contained sufficient such that no extra water neededto be added. The particle size of the ceramic fell in the range 120 to200 microns; for smaller particulate material, where the total surfacearea is greater, more of the silylating agent is required.

We have found that carrying out the silylation process under a vacuumcan avoid undesirable porosity.

Epoxide Coating of Aminopropylsilyated Ceramic Powder

Aminopropysilyated K200 powder [as prepared in the preceding example](30 g) was added to a mechanically stirred solution of the epoxy moietyof bipartite “two pack” EpoFix (from Struers, UK) epoxy resin system (3g) in dimethylformamide (150 ml). The mixture was stirred for 16 h andthe solid was collected and washed successively with dimethylformamide(2 times 50 ml), and chloroform (3 times 50 ml). The powder was allowedto dry in air (fume extracted cabinet) for several hours (typically 6h).

Method of Manufacture of a Ceramic/Epoxy Composite

Lightly ground (mortar and pestle) ceramic powder (8.4 g) was placed ina cylindrical silicone mould (36 mm diameter and deeper than 1 cm). Thepowder was lightly compressed into a parallel sided disk shape byapplication of a stainless steel disc and hand pressure to the topsurface. Prepared and degassed (10 min) EpoFix (Struers, UK) resin (3.5g) was placed on the top surface and the mould was then placed in avacuum dessicator. The dessicator was gradually evacuated of air and asthe pressure within the dessicator reduced the ceramic and the resinbegan to degas. (Care needs to be taken to ensure that the degassing isnot too vigorous or the resin and some ceramic are forced out of themould.) The degassing was controlled by admission of small amounts ofair as needed. After a few minutes the degassing process became lessvigorous and the dessicator was evacuated to the best that could beobtained from the vacuum pump (less than 0.05 mm Hg). Total degassingtime was 40 minutes, including the initial 10 minute degas of the resinprior to its addition to the mould. The dessicator was vented with airand the mould (together with its content) was transferred to an oven at100 C and allowed to cure for 1 h. After being allowed to cool, thecomposite is then ready for machining.

Note that it is possible to allow the resin to cure at lowertemperatures; for example 24 h is needed for a full cure at roomtemperature. Curing the composite at 100 C at elevated pressure (in anautoclave at (typically) 40 psi and as high as 90 psi) furnished aproduct with a higher dielectric constant and an increased electricalbreakdown strength. Autoclaving also suppressed void formation. Forlarge samples (e.g. 60 mm in diameter, 50 mm deep) it is advised that adeep mould (e.g. 100 mm) having an inverted conical section in its upperhalf (as shown in photo/diagram) is used; that design prevents resinspilling from the mould should the degassing stage become too vigorous.

The BDS and permittivity of the materials made by methods A to E and C′are shown in FIG. 7. The silylation has improved the BDS significantlyand we now have values of BDS approaching 25 with high permittivityvalues of close to 30. Also shown in FIG. 7 are results for compositematerial made by Method C incorporating the additional autoclaving step,composite material made by method C′, incorporating the additionalautoclaving step, and by method C′ with an epoxy shell added to thesilylated ceramic particles and also incorporating the additionalautoclaving step.

The three sets of data for the methods including autoclaving producedhigher permittivities than all any of the other methods, with comparablyhigh break-down strengths. The best results were from MethodC′+autoclaving, next were Method C′+epoxy shells+autoclaving, and nextMethod C+autoclaving.

Basic four point bend mechanical tests were undertaken in order to get afeel for the toughness of the new composite materials. High quality thinrectangular samples (30 mm by 10 mm by 0.6 mm) of the neat K200 ceramic(from block), the non-silylated K200 powder/epoxy composite (made bymethod C) and the aminosilylated K200 powder/epoxy composite (made bymethod C′) were prepared using standard cutting and lapping techniquesand tested using an Instron 4507 universal tension and compressionmechanical testing machine.

The results of the 4-point bend tests are summarised in FIG. 8. Althoughthe neat ceramic has a greater ultimate flexural strength, its fracturetoughness is much less than that found for the composite samples. Thearea under the load/displacement curve is a measure of the absorbedenergy density (see FIG. 9). FIGS. 8 and 9, and Table 1 below, show thatthe composite is more deformation tolerant than the ceramic. Thematerial is also more damage tolerant. In addition, the effect ofsilylating the ceramic powder prior to incorporation into a compositefurther increases the deformation tolerance of the final body (i.e. thearea under the curve in FIG. 8 is increased).

TABLE 1 Mechanical properties determined by a four-point bending test.Ultimate strain (to Energy absorbed per unit Material failure) volume(to failure) K200 ceramic 0.024%  30.8 kJ/m³ Composite (made by 0.57% 231 kJ/m³ Method C) Silylated composite (made 0.94% 449.1 kJ/m³  bymethod C′)

A simple, qualitative experiment was designed to demonstrate thesuperior toughness of the ceramic/epoxy composite compared with the neatceramic material. A test sample (600 microns thick) was placed at thebottom of a vertical plastic pipe that acted as a guide for a stainlesssteel, spherical weight (6.7 g) that was dropped from the top end of thepipe (1.2 m above the sample).

When the weight struck a neat ceramic sample coupon, the couponshattered into many pieces. In contrast, when the weight struck theepoxy/ceramic composite sample coupon (made by method C′), the sampledid not shatter but remained whole; there was no visual evidence ofdamage.

The samples we had tested to this point had been bulk samples in thesense of being much thicker than thin-film materials, but had beenrelatively thin, of the order of 0.6 mm. We next investigated scaling-upthe sample thickness in order to provide samples useful in applicationsrequiring such relatively thick samples.

In order to avoid problems with degassing larger samples, we fabricatedsome thick samples in layers.

51.4 g powder K200 and 18.36 g resin was used in total to make a first,two-layer sample, with each layer consisting of 25.7 g powder and 9.18 gresin. The resin was made in two batches when required, rather than thewhole amount being made at once, because the resin deteriorates quickly.For each batch, we made up a small quantity of resin i.e. 12.5 gresin/1.5 g hardener, and out of that used 9.18 g, the rest being thrownaway.

The resin and hardener were mixed then degassed for 15 minutes.

Half the powder was placed in a mould and compacted by hand, and thenhalf the resin was added. The mixture was degassed for 30 minutes, letto air, and then the excess resin was removed. (Half-way through thesecond degas the second batch of resin was made up.)

The remaining powder was added and compacted as much as possible,although care was taken to avoid resin left on the surface of theprevious layer being squeezed up the sides of the mould onto the top ofthe new powder. Resin was added. The whole mixture was degassed for 30mins and cured in the oven at 95 C. Any excess resin was machined/groundoff after the curing.

Each layer had a thickness of slightly more than 10 mm thickness; thefinal thickness when machined was 20 mm. The samples had a 35 mmdiameter.

Further samples were produced by the same method but with the curingbeing in an autoclave at 95 C for 45 minutes under air at a pressure of50 psi.

Samples that were thicker still (greater than 20 mm) were produced bybuilding up more than two layers.

The powder to resin ratio in each layer was determined by scaling upfrom the thinner, 3 mm samples. Any excess resin that had not been drawninto the powder after the 30 minute degas was then removed. That wasdone at each layer, apart from the last layer.

For a six-layer sample, we achieved a thickness of 55 mm total, whichwas machined to 40 mm. The diameter was 63 mm.

We also manufactured a sample with a metal tube to serve as anelectrode. The sample consisted of 6 layers with 50.4 g powder, 18 gresin for each layer, and had a 60 mm diameter.

The thick samples made by the layering method had permittivities andbreakdown strengths similar to those of their thinner counterparts. Thelayered, thick samples were also found to have improved breakdownstrength (compared with unlayered, thick samples) under an applied ACvoltage (previous tests had used a DC voltage).

We also manufactured thicker samples using ‘one-shot’ vacuum casting,which allows the fabrication of composite structures without using thelayering process described above. In one example, K200 powder was packedinto a mould, degassed under vacuum and had a very low viscositypolyeurethane resin poured on top. Subsequent application of pneumaticpressure ensured that the powder was completely wetted and impregnatedwith resin. The blocks produced had a castellated structure at one end.

In a further example a rod of K200 ceramic/polyurethane composite wassuccessfully produced with length greater than 150 mm and diameter 15mm.

We also incorporated transition metal chelates into the resin of thecomposites. Such materials tend to decompose under the conditions ofresin cure to furnish polar species that should increase thepermittivity. Preliminary results on this approach suggest that thematrix permittivity could be increased by a factor of 3 withoutadversely affecting the electrical breakdown strength.

Initial studies we have undertaken (FIG. 10) on alternative ceramics toK200 have shown that lead magnesium niobate may be a better material.FIG. 10 also shows an early result using Morgan Matroc K1700 material.The K1700 has been milled from a block and added to the matrix in thesame manner as the K200.

Typically, higher viscosity resins have a higher permittivity. Furtherexamples of materials that may be used as the matrix material includeDR001 Rigid Polyurethane or DR014 Casting Rigid Polyurathane (bothavailable from Atlas Polymers Limited, Units 13-15 Cambrian IndustrialEstate, Coedcae lane, L1antrisant, CF72 9EW, UK), or EPO-NAN-YA,NPER-032 or EPO-NAN-YA, NPEL-128 (both products of Nan Ya PlasticsCorporation, Taiwan, and obtainable from Whyte Chemicals Limited,Marlborough House, 298 Regents Park Road, Finchley, London, N3 2UA).

The Nan Ya products NPER-032 and NPEL-128 are each an epoxy moiety of abipartite resin system; thus a hardener is required in order to obtain acured resin system. We used an amine hardener, EpoFix amine hardener,but other hardeners would be suitable, including other diamines,triamines and anhydrides. In one example, 25 g of NPEL-128 was mixedwith 3 g of EpoFix hardener. After de-gassing for 30 min. the resin wascured for 1 h at 100 C.

NPEL-128 is a very viscous liquid whereas NPER-032 has a low viscosity.Whilst each can be used individually, the best quality and bestperforming ceramic composites are prepared from mixtures of the tworesins together with a hardener. In one example, a mixture of 12.5 g ofNPEL-128, 12.5 g of NPER-032 and 3 g of EpoFix amine hardener was usedto impregnate 8.4 g of K200 ceramic powder.

We have thus demonstrated several complementary methods that provide lowloss, high dielectric constant materials with high electrical breakdownstrength and mechanical integrity. Through an understanding of some ofthe mechanisms that contribute to the electrical strength and lossfeatures, e.g. percolation, homogeneity, voids, etc. we have been ableto design a material system that demonstrates breakdown strength atleast 3 times greater than the analogous K200 ceramic and almostapproaching the breakdown strength of the host resin. We havedemonstrated that composites are fourteen times more deformationtolerant (i.e. energy absorbing) than bulk ceramic, have significantlyenhanced fracture toughness, and have strain to failure improvement of40 times.

In still further experiments, we have recently achieved still betterresults. Samples made using method C using K1700 ceramic and K3500ceramic (in an epoxy resin matrix) gave the following values:—

Breakdown Modulus Loss Strength MATERIAL Permittivity Tangent (kV/mm) ReIm K1700/epoxy 83.65 0.022 10.6 83.62 1.84 K1700/epoxy 85.07 0.023 15.485.04 1.96 K3500/epoxy 120 0.025 6.5 119.96 2.99 K3500/epoxy 101 0.02510.5 100.97 2.5

We believe that the relatively large difference between the data fromsamples of the same composition can be explained by the fact that thesamples were prepared from block form and crushed using a mortal andpestle for 30 minutes and that, in this preliminary experiment, we madeno attempt to fine grind the particles or separate the particles bysizing or the like.

Various methods have been described above for producing a dielectricmaterial according to the invention. The methods are summarised in Table2. Table 3 summarises the properties of the example dielectric materialsproduced by each example method.

Of course, in the light of knowledge of the invention, many othermethods of making the dielectric material will suggest themselves to theskilled person. Similarly, many other materials will suggest themselvesfor use in the methods. Whilst the present invention has been describedand illustrated with reference to particular embodiments, it will beappreciated by those of ordinary skill in the art that the inventionlends itself to many different variations not specifically illustratedherein. For that reason, reference should be made to the claims fordetermining the true scope of the present invention.

TABLE 2 Summary of example methods described herein Evacuation topromote resin Silyla- Epoxy Auto- Method Mixing Compression infiltrationCuring tion shell claving A Filler material Mixture No 1 hour No No Nomixed with compacted at @100 C. matrix 1.25 GPa material precursors,diluted with solvent, solvent evaporated. B As A, but As A No As A No NoNo mixing at 60 C. C Filler material Filler elements Yes At room No NoNo compacted, compacted by temp degassed hand matrix material precursorsadded. D Filler material No Yes As C No No No and degassed matrixmaterial precursors mixed. E Filler material Mixture Yes As C No No Nocompacted, compacted at resin added 1.25 GPa under compaction and theninfiltrated under vacuum C′ As C As C Yes As C Yes No No C + As C As CYes As per No No Yes auto- auto claving clave cycle C′ + As C As C YesAs per Yes No Yes auto- auto claving clave cycle C′ + As C — Yes As perYes Yes Yes epoxy auto shell + clave auto- cycle claving

TABLE 3 Summary of results from the example methods described herein.Max real Min imaginary Max breakdown Method permittivity permittivitystrength (kV/mm) A (not an embodiment 53.5 3.21 3.5 of the invention) B23 0.2 20.3 C 26 0.5 22 D 25 0.5 21.5 E 21 0.5 21.5 C′ 29 0.46 24 C +auto-claving 39 0.06 18 C′+ auto-claving 45 0.66 None obtained C′+ epoxyshell + 41 0.62 20.2. auto-claving C′+ epoxy shell 35 0.55 23.2

1. A method of producing a bulk dielectric material having, at afrequency greater than 1 MHz, (i) a permittivity having a real part ofmagnitude greater than 10 and an imaginary part of magnitude less than3, and (ii) an electrical breakdown strength greater than 5 kV/mm, themethod comprising distributing a particulate filler material in a matrixmaterial to form a solid composite material having a minimum dimensiongreater than 2 mm, wherein the method comprises placing a mixture of thefiller material and the matrix material within a container andevacuating the container.
 2. The method as claimed in claim 1, furthercomprising the step of mixing the filler material with the matrixmaterial at a temperature above room temperature.
 3. The method asclaimed in claim 1, further comprising the step of compressing a mixtureof the filler material and the matrix material.
 4. The method as claimedin claim 3, wherein the mixture is compressed at a pressure of at least1 GPa.
 5. The method as claimed in claim 1, further comprising a step ofproviding the particles of the filler material with a coating.
 6. Themethod as claimed in claim 5, wherein the coating is a modified surface.7. The method as claimed in claim 6, wherein the coating is anorganosilicate shell.
 8. The method as claimed in claim 7, wherein thecoating is applied by silylation.
 9. The method as claimed in claim 5,wherein the coating is an epoxy shell.
 10. The method as claimed inclaim 5, wherein the coating is a metal or a metal oxide.
 11. The methodas claimed in claim 5, wherein the coating is applied under a vacuum.12. The method as claimed in claim 5, wherein the particles of thefiller material contain pores and the coating penetrates into the pores.13. The method as claimed in claim 1, wherein the bulk dielectricmaterial is built up in a plurality of layers.
 14. The method as claimedin claim 13, wherein each of the layers has a minimum dimension greaterthan 5 mm.
 15. The method as claimed in claim 1, wherein the bulkdielectric material is cast in a mold having a minimum dimension greaterthan 5 mm.
 16. The method as claimed in claim 15 wherein the bulkdielectric material is cast in a single step.